In U.S. Pat. No. 3,842,194 issued Oct. 15, 1974 Jon K. Clemens, a video playback system of a variable, capacitance form is disclosed. In one configuration of the Clemens system, information, representative of recorded picture and sound, is encoded in a relatively fine spiral groove on the surface of a disc record (e.g., groove width--3.5 micrometers, groove depth--1.0 micrometers). The record substrate is covered with a coating of conductive material and a dielectric layer overlying the conductive coating. During playback, a pickup stylus, having a thin conductive electrode (e.g., about 0.2 micrometers thick), engages the groove as the record is rotated by a supporting turntable. Capacitive variations between the stylus electrode and the conductive coating are sensed to recover the prerecorded information.
In the systems of the above type, the use of a relatively fine record groove and the groove-engaging requirement for the pickup stylus result in a stylus tip which is extremely small.
Typically, the stylus support tip is defined by a prow, a substantially flat, V-shaped rear surface remote from the prow, a bottom surface extending from the bottom edge of the rear surface and a pair of side surfaces extending from the side edges of the rear surface and converging at the prow. The intersection of the rear and the side surfaces with the bottom surface forms, for example, a triangular footprint. Illustratively, the angle included between the two side edges is equal to 42.degree., the length of the bottom edge is about 2 micrometers, the height of the triangular footprint is about 4 micrometers and the angle between the prow and the rear surface is about 40.degree..
In the previously mentioned Clemens patent, two alternative forms of stylus structure are disclosed: (1) a symmetrical structure, wherein the conductive stylus electrode is "sandwiched" between dielectric support material, which extends ahead of, and behind, the electrode in symmetrical fashion (FIG. 1); and (2) an "asymmetrical" structure, wherein the stylus electrode comprises a conductive coating on the rear surface of a dielectric support (FIG. 5).
The asymmetrical stylus form has proven to be convenient for ease in stylus fabrication, in that the entire rear surface is coated with conductive material. However, the stylus electrode, being V-shaped, gets wider as the stylus structure wears out. As the stylus electrode gets wider, the spurious capacitance between the electrode and the conductive coating in the adjacent grooves becomes significant, introducing noise in the recovered signal. When the stylus electrode becomes wider than the groove width (e.g., 3.5 micrometers), it is desirable to replace the stylus.
Moreover, as both the stylus structures disclosed in Clemens get wider with the use, the used styli simultaneously cover several grooves during playback, which may introduce stylus mistracking and resultant signal distortion.